Biogas Operating Manual
Page 12
Geotechnical Instruments (UK) Ltd
BMUK0001 / Issue 1
The Anemometer reads in both velocity and flow rate. In order to calculate the volume flow rate the
pipe diameter will need to be entered into the instrument via the instrument settings screen. The
anemometer can be zeroed via the gas calibration screen.
Please Note…The anemometer probe is NOT part of the analyser Ex certification and is therefore
NOT certified for use in a potentially explosive atmosphere.
4.6 Cross-Gas
Effects
Methane, Carbon Dioxide and Oxygen
Methane is measured using dual beam infra-red absorption. Your instrument is calibrated using
certified methane mixtures and will give correct readings provided there are no other hydrocarbon
gasses present within the sample (e.g. ethane, propane, butane, etc.). If there are other hydrocarbons
present, the methane reading will be higher (never lower) than the actual methane concentration being
monitored.
The extent to which the methane reading is affected depends upon the concentration of the methane in
the sample and the concentration of the other hydrocarbons. The effect is totally non-linear and
difficult to predict.
Carbon Dioxide is measured by infra-red absorption at a wavelength specific to carbon dioxide.
Therefore, the carbon dioxide reading will not be affected by any other gases usually found on landfill
sites.
The infrared sensors will not be "poisoned" by other hydrocarbons and will revert to normal operation
as soon as the gas sample has been ‘purged’.
The oxygen sensor is a galvanic cell type and suffers virtually no influence from CO
2
, CO, H
2
S, NO
2
,
SO
2
or H
2
, unlike many other types of oxygen cell.
H
2
S Measurement
H
2
S measurement via the external gas pod could be affected by other gases. The main cross gas
effects are:
•
SO
2
:
20% effect
•
NO
2
:
20% effect
Other cross sensitivities are possible. If you suspect a cross sensitivity problem please contact you
supplier for additional information.
